GB1578625A

GB1578625A - Bimodal cavity resonator beam position monitor

Info

Publication number: GB1578625A
Application number: GB6713/77A
Authority: GB
Original assignee: Atomic Energy of Canada Ltd AECL
Current assignee: Atomic Energy of Canada Ltd AECL
Priority date: 1976-05-17
Filing date: 1977-02-17
Publication date: 1980-11-05
Also published as: FR2357914B1; CA1044373A; JPS52139357A; AU2251477A; DE2706631B2; DE2706631C3; DE2706631A1; FR2357914A1; US4060762A

Description

PATENT SPECIFICATION

( 11) 1 578 625 Application No 6713/77 ( 22) Filed 17 Feb 1977 Convention Application No 252671 ( 32) Filed 17 May 1976 in Canada (CA)

Complete Specification Published 5 Nov 1980

GOB 7/14 Index at Acceptance Gi N 19 B 2 X 19 C 7 19 F 7 B 19 H 7 X ( 54) BIMODAL CAVITY RESONATOR BEAM POSITION MONITOR ( 71) We, ATOMIC ENERGY OF CANADA LIMITED, a Company incorporated pursuant to the Atomic Energy Control Act, Chapter 11, Revised Statutes of Canada, 1952, as amended, and having its head office in the City of Ottawa, Province of Ontario, Canada, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:The invention is directed to a charged particle beam position monitor and in particular to a non-intercepting monitor using a single bimodal resonant cavity in which uncoupled orthogonal modes are simultaneously excited by the particle beam.

In the operating of beam current devices such as charged particle accelerators, it is highly desirable to monitor the position of the beam generated to allow functions such as focusing of the beam to be achieved with high accuracy Also fast shut-down systems require information quickly if a power beam starts to drift off course Most devices presently in use for determining beam position are of the intercepting type which tend to perturb the beam during the monitoring process Many provide information only on the beam halo which surrounds the hard core of the beam In addition, in monitoring high power beams, heat dissipation, the emission of unwanted radiation and activation of components can create problems.

The present invention provides a charged particle beam monitor which consists of a bimodal cavity, resonant at the beam frequency or a harmonic thereof.

The cavity includes beam holes, which are concentric with the z-axis passing through the x and y axes of the cavity, on opposite surfaces to allow the particle beam to pass through the cavity to excite the cavity in a first mode if the beam is displaced along the x-axis and in a second mode orthogonal to the first if the beam is displaced along the y-axis The cavity further includes tuning means, such as capacitive tuning screws mounted in the cavity wall at the potential maxima of the modes, to tune the two modes to the same frequency, Detectors such as magnetic probes symmetrically positioned around the circumference of the cavity on the x and y axes, are coupled to the fields in the two modes and provide signals which are a function of the displacement of the centroid of the beam along the x and y axes respectively The x and y coordinates of the beam centroid may be provided by determining the average amplitude and phase of the probe signals for the x-axis and the y-axis respectively, and converting the amplitudes to distances along the axes using a Bessel function.

In the drawings:

Figures la and lb illustrate the TM 11 o mode oscillation in a right circular cavity; Figure 2 illustrates the breakup of the TM 110 mode in an elliptical cavity; Figure 3 illustrates the beam position monitor in accordance with this invention; Figure 4 illustrates a cross-section of the monitor in Figure 3; Figures 5 a and 5 b illustrate the modes excited in the monitor depending on the displacemement of the beam; and Figure 6 illustrates the coordinate determining circuitry.

RF oscillations in a cavity resonator can occur in many ways with each type of oscillation having particular resonant frequency The representations of the magnetic H and electric E field distributions of a

TM 110 mode of a right circular cylindrical cavity 1 are shown in Figures la and lb A charge particle beam travelling slightly off the axis 2 through the cavity 1 can excite this mode by interaction with the electric field of ( 21) ( 31) ( 33) ( 44) ( 51) ( 52) xn ell ( 19) 1 578 625 the mode With sufficient beam current, the cavity fields can be built up to such a level that beam blow-up results due to the high magnetic field on axis 2, as described in the reference: Gluckstern, R L, Proc of MURA Conf on Linear Acc, BNLAADD-38 ( 1964) 186.

In the TM 110 mode the electric field is zero on axis 2 and its direction changes sign across the plane of symmetry Also it increases to a maximum at 0 44 R where R is the cylinder radius of the cavity 1 When the beam current is known the amplitude of the cavity oscillations can be interpreted to give the displacement from the plane of symmetry while the phase will give the sign of the displacement.

In practice, any departure from azimuthal symmetry represents a perturbation to the cavity fields causing modes to split into components having similar field patterns but different resonant frequencies As described in the reference Chu, L J, Journal of Applied Physics 9 ( 1938) 583, Chu has shown that for an elliptical cavity, modes with azimuthal asymmetry in their field distributions will break up into two orthogonal component modes having different resonant frequencies The magnetic field lines 22 and 23 for the two orthogonal TM 1 o like modes in an elliptical cavity 21, are represented in Figure 2 The figure shows that the magnetic field lines 22 and 23 close to the axis of the cavity 21 orient themselves along the major or x-axis and minor or y-axis, respectively.

Maximum electric field positions for the major and minor modes occur at positions A and B respectively Using the criteria published by Slater in the reference: Slater, J.C, Microwave Electronics, Van Nostrand, Princeton, N J ( 1950) p 81, it is possible to introduce a tuning plunger, at positions A and B which will lower the resonant frequency of each mode independently This principle has been used in paramagnetic resonance studies in diamond to continuously split the degenerate TM 1 o modes over a continuous frequency range, as described in the reference: Sorokin, P P.

et al, Physical Review 118, no 4 ( 1960) pages 939 to 945.

The preferred embodiment of the beam monitor in accordance with this invention is illustrated in figures 3 and 4 The monitor includes a bimodal right circular cavity 31 having a first and a second pair of symmetrically arranged capacitive tuning screws 32 x and 32 y projecting into the cavity 31 The first pair of tuning screws 32 x are positioned on either side of the cavity axis 33 on the x-axis at the electric field maxima of a first cavity mode having magnetic field lines 34.

The second pair of tuning screws 32 y are positioned on either side of the cavity axis 33 on the y-axis at the electric field maxima of a second cavity mode which is orthogonal to the first mode and has magnetic field lines

The tuners 32 x, 32 y may be made of stainless steel and made to penetrate the cavity through glass windows to protect the vacuum integrity of the cavity 31.

The beam monitor also includes beam holes 36 and 37 on the front and back surfaces respectively of the cavity 31 to allow passage of the charged particle beam to be monitored through the cavity 31, the beam holes 36 and 37 being concentric with the cavity axis 33 In order to further maintain the vacuum integrity of the cavity 31, a first beam pipe 38 may be used to connect the cavity 31 to the beam accelerating system and a second beam pipe 39 may be used to connect the cavity 31 to a utilization means, such as a target or another accelerating structure.

The beam monitor further includes four symmetrically positioned magnetic probes 40, 41, 42 and 43 around the outer circumference of cavity 31 These probes having a housing 44, 45, 46 and 47 to maintain the vacuum in cavity 31 Probes 40 and 41 are preferably positioned on the x-axis such that only magnetic lines 34 will be coupled to these probes, while probes 42 and 43 are positioned on the y-axis to couple orthogonal magnetic lines 35 Each probe is thus oriented to strongly couple to one mode but not to the orthogonal mode, hence an opposite pair of probes 40 and 41 is isolated from the other pair of probes 42 and 43, with isolation between the modes being greater than 40 db.

In operation, the cavity 31 is constructed to be resonant at the accelerator frequency or at a harmonic thereof and both modes of the cavity 31 are tuned to the same frequency by tuning screws 32 x and 32 y.

As shown is Figures 5 a and 5 b, a charged particle beam 30 passing through the bimodal cavity 31 will excite the modes appropriate to the cartesian coordinates of the beam centroid Thus if the beam 30 is displaced along the x-axis as shown in Figure 5 a, one mode of the cavity 31 will be excited and will be detected by probes 40 and 41 If the centroid of the beam 30 is to the left of the y-axis, the same mode will be excited with the electrical field lines E and magnetic field lines H running in directions opposite to those shown Probes 40 and 41 are coupled to an amplitude measuring circuit 61 in the beam coordinates determining circuit shown in Figure 6 Circuit 61 provides an output S, which is the average of the amplitudes of the signals from probes 40 and 41 In addition, one of the probes 40 or 41, in this instance probe 41, is coupled to a phase determining circuit 62 which provides a plus or minus output signal kx depending on the phase of 1 578 625 the probe 41 signal with respect to a reference This plus or minus signal indicates whether the beam is to the left or the right of the y-axis The outputs from circuits 61 and 62 are coupled to a coordinate calculating circuit 63 Circuit 63 calculates the position x of the centroid of beam 30 using the equation:

x = 0 037 + 0 385 for greater accuracy either the J 1 Bessel function or the following expression may be used:

x = 0 00039 + 0 507 S O 01952 0.04753 Thus circuit 63 provides both the distance and direction of the centroid of the beam 30 along the x-axis.

Similarly, if the beam 30 is displaced along the y-axis as shown in Figure 5 b, the orthogonal mode of the cavity 31 will be excited and will be detected by probes 42 and 43 Probes 42 and 43 are coupled to an amplitude measuring circuit 61 y, with probe 42 additionally coupled to a phase determining circuit 62 y Circuits 61 y and 62 y are identical to circuits 61 x and 62 x respectively, and provide output signals SY and 4)y which are coupled to a y-coordinate calculating circuit 63 y which is identical to circuit 63 x.

Generally, the centroid of the beam 30 will be displaced both along the x-axis and the y-axis, and thus both orthogonal modes will be excited simultaneously and the x and y coordinates of the beam 30 will be simultaneously provided by circuits 63 x and 63 y.

By constructing the cavity 31 at a harmonic of the accelerator frequency, information on the phase distribution of the bunched particle beam 30 may be obtained If the accelerator generating the beam 30 is expected to have beam bunches of width less than 600 the cavity may be constructed to resonate at the third harmonic Thus if the phase distribution of the beam 30 is 1200, then the monitor would be very insensitive to the third harmonic but sensitive to the second harmonic The maximum sensitivity will be achieved when no = n where N is the harmonic number and Q represents the limits of phase at the fundamental frequency For example, if the beam 30 to be monitored is generated by an accelerator at a frequency of 805 M Hz and the bunched beam is distributed in longitudinal phase space over a range of 600 at 805 M Hz, to obtain maximum sensitivity the cavity 31 would be constructed to resonate at 2,415 M Hz or the third harmonic Such a cavity 31 could be 14 75 cms in diameter and 6 cms long with beam holes having a diameter of 3 5 cms.

Claims

WHAT WE CLAIM IS

1 A charged particle beam position monitor comprising:

resonant cavity means capable of being excited in a first mode about the x-axis of the cavity means and in a second mode orthogonal to the first mode about the y-axis of the cavity means, said cavity means having beam holes in opposite surfaces of said cavity means, said beam holes being concentric with a z-axis through the intersection of the x-axis and y-axis, to allow the particle beam to pass through said cavity means such said first mode is excited in the cavity when the centroid of the beam is displaced along the x-axis and said second mode is excited in the cavity when the centroid of the beam is displaced along the y-axis; tuning means mounted on said cavity means for tuning the orthogonal modes to the same frequency; first detector means coupled to said first mode in said cavity means to provide a signal as a function of beam displacement along the x-axis; and second detector means coupled to said second mode in said cavity means to provide a signal as a function of beam displacement along the y-axis.

2 A beam position monitor as claimed in claim 1 wherein said first and second detector means each include two magnetic probes, the first detector probes being located at opposite sides of said cavity on the x-axis and the second detector probes being located at opposite sides of said cavity on the y-axis.

3 A beam position monitor as claimed in claim 2 which further includes:

first amplitude measuring means coupled to said first detector probes for determining the average amplitude of the signals from the first detector probes; first phase measuring means coupled to one of said first detector probes for determining the phase of the signal from the first detector probe; first means coupled to said amplitude measuring means and first phase measuring means for providing the x coordinate of the beam; second amplitude measuring means coupled to said second detector probes for determining the average amplitude of the signals from the second detector probes; second phase measuring means coupled to one of said second detector probes for determining the phase of the signal from the second detector probe; and second means coupled to said second amplitude measuring means and said second phase measuring means for providing the y coordinate of the beam.

4 A position monitor as claimed in 1 578 625 claim 1 wherein said tuning means includes four capacitive tuning screws positioned at the electric field maxima of said orthogonal modes.

5 A position monitor as claimed in claim 4 wherein said cavity means consists of a right circular cavity resonant at a harmonic of particle beam frequency.

6 A charged particle beam position monitor substantially as described herein with reference to, as as shown in Figures 3, 4 and 6 of the accompanying drawings.

ABEL & IMRAY, Chartered Patent Agents, 303-306 High Holborn, Northumberland House, London, WC 1 V 7 LH.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1980.

Published by The Patent Office, 25 Southampton Buildings, London WC 2 A t AY, from which copies may be obtained.